High-Speed Packet Access (HSPA) is a collection of mobile telephony protocols that extend and improve the performance of existing Universal Mobile Telephony System (UMTS) protocols. Two standards, High-Speed Downlink Packet Access and High-Speed Uplink Packet Access also referred to as Enhanced Uplink (EUL) have been established. The enhanced uplink introduces a new transport channel, the Enhanced Dedicated Channel (E-DCH). A dedicated channel (DCH) is assigned to only one UE at a time. The DCHs are power controlled which implies that the transmitter power is increased if the channel is too poor and the power is reduced if an unnecessary high power level is used.
At the physical layer, the Enhanced Uplink introduces e.g. the E-DCH Dedicated Physical Control Channel (E-DPCCH) and the E-DCH Dedicated Physical Data Channel (E-DPDCH). The E-DPDCH is used to carry the E-DCH transport channel and the E-DPCCH is used to carry the control information associated with the E-DCH such as the E-DCH Transport Format Combination Indicator (E-TFCI). The Dedicated Physical Control Channel DPCCH is used to carry pilot symbols used for channel estimation.
To increase the data rate in the uplink, higher order modulation (HOM) based on 16 QAM (Quadrature Amplitude Modulation) is introduced to the uplink E-DCH. The introduction of 16 QAM doubles the data rate with respect to Release 6 of the 3GPP specifications concerning Enhanced Uplink and allows peak data rates up to 11.5 Mbps (with coding rate equal 1). The transmission power of the data channel, E-DPDCH, depends on the transport format used and is adjusted relative to the DPCCH power. The DPCCH power is set by the inner power control loop to reach the SIR target set by the outer loop power control.
The Open loop power control is the ability of the User Equipment (UE) transmitter to set its output power to a specific value. It is used for setting initial uplink and downlink transmission powers when a UE is accessing the network. The Inner loop power control (also called fast closed loop power control) in the uplink is the ability of the UE transmitter to adjust its output power in accordance with one or more Transmit Power Control (TPC) commands received in the downlink, in order to keep the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target.
Reliable demodulation of high rate signals requires a good phase reference (by using pilot symbols for channel estimation. It has been shown that the current power settings in Release 6 of the 3GPP specifications are not sufficient to guarantee good performance. A better phase reference can be obtained by scaling the control channel (DPCCH) power according to the transport block size which indicates the current bit rate, wherein the transport blocks are transmitted by the E-DPDCH. The DPCCH is then transmitted at higher power for high data rate transmission. The DPCCH carries pilot symbols that are used as a phase reference for channel estimation as illustrated in FIG. 1.
FIG. 1 illustrates a network comprising a plurality of radio base stations 110a,b,c connected to a radio network controller (RNC) 100. The radio base stations 110a,b,c are adapted to communicate wirelessly with the UEs 120 (only one UE is illustrated). One UE 120 may be connected to more than one radio base station simultaneously referred to as soft handover (SHO) as illustrated in FIG. 1.
Assume that the boosting of the DPCCH transmission power when high data rates are transmitted is applied. A problem with the power control loop arises when the UE is in SHO. Consider the case when the UE 120 is in SHO with a first base station 110a and a second base station 110b. The first base station 110a is the serving base station, i.e. the first base station 110a is responsible for the scheduling of the user. The UE 120 increases the power of DPCCH according to the transmission data rate negotiated with the first base station 110a. The second base station 110b has no knowledge that the UE 120 has boosted its power and the SIR target at the second base station 110b will be set at a value lower than the correct value. The power control loop with the second base station 110b then will react to the increased received DPCCH power by sending “down” power commands to the UE 120. Since the UE 120 listens to the power control commands of both base stations 110a,b and acts according to the “OR of the down commands”, the power is lowered as soon as at least one TPC indicates a lower power. Thus, the UE 120 will lower the transmitted power even when the serving base station commands otherwise.
Hence, the problem is the generation of incorrect power control commands sent by the non-serving base station which is not aware of that the UE has boosted the DPCCH power. This leads to a too low receive power, and to an increased probability that transport blocks cannot be correctly decoded. Hence the system capacity is degraded.
To address this problem, it has been proposed that the UE should not act on the power control commands from the non-serving base stations for 2 or 3 time slots when boosting or lowering the power of DPCCH according to the granted rate. Or, according to an alternative solution, the UE should not act on any of the received power control commands for a few slots if the boosting of DPCCH is set according to the actual transmitted rate. The drawbacks of these proposals is that the convergence time for the SIR target value may be longer than the required time for the UE to ignore its power commands according to the prior art solution above. Furthermore, the power control procedure of these proposals is user dependent and can create instability in the system.